In a wide range of scientific, industrial, and medical applications, direct or inferential measurements of the contact angle between a free liquid surface and a solid interface are of great practical importance and value. Given the contact angle at a given liquid-solid interface, it is often possible to utilize known empirical laws to understand and predict a wide range of related physical phenomena. Specifically, the interfacial contact angle, along with fluid properties such as surface tension, viscosity, and density, enable modeling of processes including surface wetting, wicking in pores, and capillary pressure, to name a few. In addition, since the contact angle between a given fluid and surface bears important information regarding affinity for other materials, surface roughness, molecular surface morphology, and the like, any related measurement can prove useful in relative comparisons of surface properties.
Among the many important uses of contact angle data, one particularly common and valuable application involves measurements which provide a relative ranking of how substances will print, bond, or adhere on a given surface. As is well known, surfaces often display varying degrees of affinity for substances which may be fused or bonded to them. Many polymer surfaces, for example, display a general lack of affinity for other materials and are inherently difficult to fuse, bond, adhere, or print with other substances. In such cases, a variety of treatments or coatings are often applied to alter surface compatibility. Measures of surface properties which correlate with the affinity of a given surface to other materials is of value in related enterprises. Contact angle measurements are commonly used for this purpose (to control and gage the effectiveness or surface treatments, for example) since they are known to correlate with surface affinity for other substances and relative compatibility. Various less direct measurements, which correlate to contact angles in some way, find similar use in ranking and comparing the properties of surfaces as they relate to printing, bonding, and adherence with other materials.
Given the vast utility of contact angle measurements, ranging from basic research to practical analysis of printability, the measurement of contact angles has been the subject of extensive investigation, development, and invention. At present, a number of techniques for the measurement of contact angle between a given fluid and surface are well known, and devices are commercially available which utilize these techniques to provide related measurements.
Among the most straightforward of techniques for measurement of contact angles involves the placement of a test droplet on a subject surface and direct observation using optical magnification. The contact angle at the edges of the drop is, thus, directly observed and measured. A wide range of commercial devices currently offer this capability including instruments manufactured by Dataphysics Instruments, GmbH of Filderstadt, Germany, Kernco Instruments co., Inc. of El Paso, Tex., and AST Products, Inc. of Bilerica, Mass.
Another technique involves mounting a subject surface on a fixture which is attached to a microbalance. The surface is then suspended within a test fluid and resulting forces are measured. After properly accounting for buoyant forces, and sample geometries, it is possible to deduce contact angle information from such measurements. In addition, it is possible to measure the angle dynamically, as the fluid surface either advances or recedes over the surface, and utilize this data to compute advancing or receding contact angle. Instruments which perform this type of analysis, thus, provide sophisticated dynamic contact able measurements for innumerable applications ranging from basic research to analysis of bonding and adherence. Any number of analytical instruments of this type are currently available commercially including those manufactured by Kruss, GmbH, Dataphysics Instruments, GmbH of Filderstadt, Germany, and AST Products, Inc. of Bilerica, Mass.
The commercial interest in such instrumentation, and the vast utility of the information they provide, has driven tremendous development in this general area. Examples of developments in the measurement of contact angles and related surface properties are exemplified by U.S. Pat. Nos. 4,050,822, 5,080,484, and 5,268,733 which are incorporated herein for reference. U.S. Pat. No. 4,050,822 describes a device which dispenses a droplet of known volume onto a test surface, and allows inference of the fluid contact angle from the measured maximum droplet height above the interface. U.S. Pat. No. 5,080,484 describes a device for measuring the contact angle through the measurement of laser light reflected from the liquid and solid surfaces along a line of contact. U.S. Pat. No. 5,268,733 describes a device which enables determination of contact angle through projection of the image of a droplet onto a screen, and use of a special protractor scale facilitating accurate determination of contact angle.
Although this prior art is sophisticated, and enables accurate determination of contact angles and related information for many applications, associated instrumentation is typically complex and expensive. Although this level of sophistication fills an important need, there is considerable demand for measurement techniques more suited to immediate analysis of surface characteristics in routine analysis. Particularly in manufacturing settings, where quality control metrics are needed for surfaces, but analysis must be carried out quickly and with minimal complexity, there is a need for improved methods. In addition, there is a constant demand for advancement in this general area of surface analysis, and any technique which offers alternatives to current methodology and instrumentation holds the potential to dramatically extend and augment current measurement capabilities.
One alternative which currently exists for immediate gross indications of surface bondability and printability is provided by examination of wetting by a series of different fluids. So-called dyne test fluids are engineered with a range of different surface tensions. These fluids are progressively swabbed or smeared on to a subject surface (sometimes with a specially designed pen), to determine those which bead and those which wet. Although this is not a direct measure of contact angle for any specific fluid on the surface, the surface tension of the fluid which will wet the surface is generally correlated with the contact angle of a given specific fluid. In addition, the result is similarly correlated with affinity for inks and other substances. U.S. Pat. No. 4,694,685, which is incorporated herein for reference, presents a set of water-based fluids with different surface energies but which are nearly identical in other respects (such as viscosity). In addition, Diversified Enterprises-ADT of Claremont, N.H., U.V. Process Supply Inc. of Chicago, Ill., and Vetaphone of Kolding, Denmark, all offer dyne test fluids or dyne test pens for use in the manner described.
Although dyne test fluids are a pragmatic and immediate means for determining surface properties, they provide only a gross and somewhat subjective measurement. In addition, contamination of the fluids, toxicity, and perishibility can present a host of issues in practical application.
For all of these reasons, there is a need for methods and devices which augment those currently available for the measurement of contact angles and related information. Where such methods and devices offer extensions of the more sophisticated aspects of current technology, they promise to extend the scope of current measurement capabilities. While, in itself, the value of such extension is clear, there Is a further need for any technology which enables simple, cost effective, and accurate measurement of surface properties in routine applications such as quality control.